The present invention relates generally to the field of motion damping systems and, more particularly, to an apparatus and method for controllably damping motion of a flexible structure subjected to a dynamic environment.
Passive auxiliary mass dampers, in which the response of a primary oscillating system is attenuated by a relatively small auxiliary mass attached to the primary system by a resilient element, have long been used for linear vibration control. The dynamic vibration neutralizer (DVN), also known as the Frahm damper, is a leading member of this class. It relies on "tuning" the auxiliary mass to generate an opposing force capable of reducing motion of the primary system under steady state conditions. Another common linear device is the Lanchester damper, which relies exclusively on dissipation of mechanical energy by a viscous dashpot.
Nonlinear auxiliary mass dampers have been used in situations involving transient and/or wide-band excitations, in which linear dampers are inefficient. One highly nonlinear device, known as an impact or "acceleration" damper, is described in Paget, A., "The Acceleration Damper", Engineering, 1934, p. 557. It consists of a solid mass constrained to oscillate unidirectionally in a container with a certain clearance. When attached to an oscillating primary system, the device employs plastic deformation, Coulomb friction and momentum transfer between masses during collision to reduce vibration of the primary system. It is essentially a highly nonlinear DVN in which the coupling element has deadspace characteristics.
Even with a relatively small auxiliary mass ratio, properly designed impact dampers are superior to DVN's in attenuating the response of structures subjected to non-stationary random excitations, such as earthquake ground motion. Small damping forces generated by the impacting mass of such a damper introduce chaos into the dynamic system response by disorganizing the orderly process of amplitude buildup. This reduces structural response drastically. However, prior art impact dampers are passive devices whose characteristics are optimized for a single set of operating conditions. As such, they cannot adequately handle wide-band random excitation. This is particularly true in applications where both the root-mean-square (rms) level and the peak levels of the primary structure response are of concern, as is the case in most structural applications.
In an effort to overcome the inability of prior devices to adapt to changing conditions, applicants developed a method for active on-line control of vibration by applying corrective pulses of mechanical energy to a vibrating structure. The method, disclosed in U.S. Pat. No. 4,429,496, is highly effective in reducing the rms response as well as the peak response of vibrating structures, even when the excitation is nonstationary wide-band random. However, it relies on the availability of substantial amounts of energy to produce control pulses on demand.
Therefore, it is desirable in many applications to provide an apparatus and method for suppressing movement of a primary system in response to a dynamic environment through expenditure of relatively small amounts of control energy.